Optical instruments projecting a beam of coherent light have been used in such applications as topographic land surveying and building construction to measure line-of-sight distances to a remote target by comparing one or more characteristics of the light beam reflected from the target with the same characteristics of the projected beam. Instruments projecting a continuous wave of coherent light typically treat the distance to be measured as a function of the frequency and phase difference between the reflected and projected light beams. In exemplary form, continuous wave optical distance measuring instruments electro-optically modulate the amplitude of a beam of light and project the beam towards a stationary remote target located a known approximate distance from the instrument. The portion of the beam reflected along a path parallel with the projected beam back to the instrument from the target, e.g., a corner cube reflector, is directed via receiving optics onto a photo-detector circuit. The phase difference between the reflected portion of the modulated beam and a reference portion is a function of the fractional distance between the instrument and the target.
In one type of instrument, the precise distance between the instrument and target is determined by combining the reflected and local beam portions at a single photo-detector. The amplitude of the signal provided by the photo-detector is determined by the intensity of the combined waveform envelopes of the local and reflected beam portions which, in turn, depend upon the phase difference between the reference and reflected beams caused by the fractional difference between their path lengths.
In another type of instrument, a laser light source with a resonant internal cavity of length L is used to generate a light beam having two frequency components approximately spaced apart by C/2L, where C is the velocity of light. One portion of the light beam is projected towards the target while a second portion is projected towards a locally positioned reference surface within an external cavity having a manually variable, calibrated cavity length. The reference beam emerging from a retro-reflector in the cavity is combined with a portion reflected from the target and the combined beam is focused upon a single photocell operated to provide a conventional square law response characteristic. Square law operation provides a quadratic signal to the incident radiation containing a component at the difference frequency proportional to the product of the amplitude of the reference and reflected beams. The precise distance between the instrument and target may be determined in a null seeking procedure by calibrating adjustments of the external cavity length, taking multiple measurements at different external cavity lengths for successive interference minima, and simultaneously solving a standard algorithm for different external cavity lengths as a function of successive minima.
Neither of these types of state of the art instruments is suitable for motion detection in such applications as monitoring the stability of a normally stationary target such as a building or a newly erected load bearing structure at a construction site, because a change in the path length between either type of instrument and a remote target only can be detected if the phase of the reference and reflected beams happen to coincide precisely. Unambiguous measurement of the distance between the instrument and a remote target may be measured with these types of instruments only by continuously dithering the external retro-reflector cavity to maintain a phase null. Furthermore, stray internal optical coupling within receiver networks in instruments using a single element for detection of both the local and reflected beams may create false interference effects, i.e., interference signals vectorially added to the reflected beam portion, and thereby provide an incorrect indication of phase difference. These errors may be small when using low powered laser transmitters but, at the higher laser powers necessary to measure distances greater than several hundred yards, false interference effects become intolerable. Laser transmitter systems which use external modulators typically require high power modulator drive signals. This makes the receiver circuit susceptable to the electro-magnetic fields generated at the transmitter, which also can cause interference effects. In currently developed systems of this type, these false signals limit the accuracy and maximum range of the measurement system.
Where the receiving network includes a local optical oscillator for heterodyning the local and reflected beams, the optical phase must be controlled between the local oscillator and the reflector signal to prevent introduction of error due to phase drift. Control of the optical phase is very difficult, however, particularly if the target is not stationary relative to the instrument (movement causes doppler shift). To avoid this difficulty, one instrument uses a dual frequency laser in its transmitting network to generate a pair of co-collimated and very closely-spaced optical signals. A pair of cubic response optical detectors in the receiving network are simultaneously exposed to a local portion and a reflected portion, respectively, of the dual frequency optical signal while modulated by a common microwave oscillator signal. The laser transmitting network in this type of system requires radio frequency modulation. The use of cubic response photo-detectors permits the receiving network to heterodyne the local and reflected optical beams while substituting an easily met requirement for controlling phase differences between the local oscillator and the heterodyned reflected beam of microwave frequencies for the more difficult requirement of controlling the phases of the local oscillator signal and reflected beam at optical frequencies. Cubic response photo-detectors are experimental devices and are not as readily available as are square law photo-detectors. Furthermore, they necessitate a more complicated receiver network using more than one local oscillator.
Accordingly, it is one object of the present invention to provide an improved optical distance measuring instrument.
It is another object to provide a simplified optical distance measuring instrument.
It is still another object to provide a more accurate optical distance measuring instrument.
It is a further object to provide an optical distance measuring instrument to automatically measure the range of a target.
It is still further object to provide an optical measuring instrument to continuously measure the fractional movement of a target.
These and other objects are achieved with an optical distance measuring instrument detecting the phase position between the difference frequency component of a composite, continuous wave optical signal of coherent light having two co-collimated longitudinal mode components. The instrument has a laser generating a composite optical signal, including co-collimated longitudinal mode components separated by an adjustable difference frequency. The signal generated is locally split into two portions and a reference beam portion is directed onto a square law photo-detector while the remainder of the signal is projected to illuminate a distant target. A reflected beam is directed onto a second square law photodetector optically isolated from the first photo-detector and the reference beam. The difference frequency components of the electrical signals provided by the photo-detectors may be applied to an radio frequency phase meter. Any change in the fractional distance between the instrument and the target will be instantly indicated by the phase meter as a shift in the phase difference between the reference and reflected difference frequency components.
Alternatively, ambiguities in measurement of the target distance may be resolved by coupling the difference frequency component of the reference signal to a frequency counter and then feeding the phase meter and frequency counter output signals to a range control computer which, by a series of iterations varying the laser cavity length, can precisely determine the distance between the instrument and target.